A trench gate type semiconductor device such as an insulated gate bipolar transistor (i.e., IGBT) and a MOS field effect transistor (i.e., MOSFET) is well known. For example, a trench gate type IGBT is disclosed in U.S. Pat. No. 6,737,705. The IGBT has a cell skipping structure. Specifically, some cell regions are periodically skipped to arrange in the IGBT so that an on-state voltage is reduced. Here, in general IGBT, multiple cell regions are serially arranged in the IGBT. Each cell region functions as an element.
In the IGBT having the cell skipping structure, the thickness of a gate insulation film in a trench is uniform, i.e., homogeneous. Further, a bottom of the trench has a large curvature radius in order to reduce electric field concentration at the bottom of the trench. When a voltage is applied to a collector by a switching operation, the electric field may be concentrated at the bottom of the trench. By increasing the curvature radius of the bottom of the trench, the electric field concentration is prevented or reduced.
Further, it is required for the IGBT to reduce a switching loss when a switch of the IGBT is turned on or when the switch is turned off so that efficiency of a device is improved. Therefore, regarding reduction of switching loss when the switch is turned off, it is considered that a gate resistance is reduced so that a cutoff speed di/dt of current becomes larger. However, when this method is used for the IGBT, the following problems occur. Specifically, when the IGBT is used for switching with large current such as 400 Ampere in order to drive an inductance load, the problems occur.
When the IGBT switches off, a voltage surge is occurred in the voltage waveform, which is different from a general IGBT voltage waveform. Therefore, a collector voltage is increased up to a maximum voltage, which is larger than a power source voltage. After that, the collector voltage is reduced, and becomes constant, which is the same as the power source voltage.
The surge voltage becomes large as the cutoff speed of the current becomes larger. Therefore, in a case where the cutoff voltage of the current in the IGBT becomes large in order to reduce the switching loss when the IGBT switches off, the surge voltage becomes larger. Further, when the surge voltage exceeds a yielding voltage of the device, and when the absorbed energy of the device exceeds a breakdown energy, the device may be damaged.
Here, this breakdown is occurred when the trench gate type IGBT has the cell skipping structure. Further, the breakdown may be occurred when an IGBT has a general construction without having the cell skipping structure. Furthermore, the breakdown may be occurred in a trench gate type MOSFET.